1. Field of the Invention
The invention relates to a dryer section for drying a fibrous web, in particular a paper, cardboard, or tissue web, in machines for the production and/or processing thereof, including several dryer groups in which the fibrous web is guided over heated drying cylinders and guiding rolls with the fibrous web being pressed by at least one belt of the corresponding drying group against the jacket surface of the drying cylinder and with the fibrous web being transferred at at least one transfer point between two idling transfer elements and with the accepting element having a higher speed than the delivering transfer element.
2. Discussion of Background Information
Due to the impact of exterior forces, such as adhesive forces on rolls and belts, centrifugal forces, as well as forces-caused by vacuums or airflow, stretching occurs in moist fibrous webs, particularly in press sections and dryer sections. These stretches can then cause the formation of folds or even breaks.
In order to support the transfer of the fibrous web between the two transfer elements and to compensate for stretching, the accepting transfer elements are frequently driven at a higher speed, i.e., tension is created at the transfer point.
Here, the transfer of the fibrous web usually occurs from the last drying cylinder of a dryer group to the drying wire of the subsequent dryer group with the drying wire at least slightly wrapping around the drying cylinder. This wrapping ensures, on the one hand, a secure transfer of the fibrous web, in particular at high web travel speeds; on the other hand, however, it limits the maximum speed difference between the transfer elements.
Therefore, the present invention creates a possibility for achieving a speed difference between the transfer elements that is as small as possible, but sufficient, at the same time, to secure a safe guidance and transfer of the fibrous web at the transfer point.
According to the present invention, the transfer elements are not pressed against each other with a speed difference at the transfer point and that they have a distance from one another of less than about 70 mm.
Squeezing of the fibrous web between the transfer elements is thus omitted, which allows higher speed differences without any negative effects on the paper quality. The distance between the transfer elements is so small, however, that a secure guidance of the fibrous web is provided.
Here, it is advantageous for the distance between the transfer elements to be larger than zero and smaller than about 35 mm at the transfer point with a speed difference.
Here, the transfer elements can be embodied as rolls, preferably as drying cylinders, and/or as belts, preferably as drying belts. If a belt is used, it is advantageous for the accepting transfer element to be embodied as a belt and to be guided at the transfer point via a suction element, preferably in the form of a suctioned roll. Here, the suction element supports the transfer and the adhesion of the fibrous web on the accepting belt.
If one transfer element is formed by a roll, the other transfer element should be a belt. This allows the sagging of the rolls, which are frequently very large, at the formation of the transfer point to be omitted. However, the belt should not wrap around the roll at the transfer point in order to minimize the effects of force on the fibrous web.
It is particularly advantageous to use the arrangement in an area of high water content, i.e., also increased stretching of the fibrous web at the beginning of the dryer section, particularly between the first and the second dryer groups.
In order to allow the creation of a speed difference, i.e., a tension, behind the dryer section as quickly as possible, the first dryer group should have only one or two drying cylinders, preferably one. Here, the speed of the accepting transfer element at the transfer point should be a maximum of about 2.5% higher, preferably about 0.5 to 2.0% higher, than the speed of the delivering transfer element between the first and the second dryer group.
The present invention is directed to a dryer section for drying a fibrous web in machines for producing and/or processing the fibrous web. The dryer section includes a plurality of dryer groups, in which each dryer group includes at least one heated dryer cylinder and at least one belt arranged to press the fibrous web against a jacket surface of the at least one heated dryer cylinder, and, wherein at least one of the dryer groups further includes guide rolls. Transfer elements, including a delivery transfer element and an accepting transfer element, are arranged to form at least one transfer point. The accepting transfer element has a greater speed than the delivering transfer element, and the transfer elements are arranged not to press against each other at the transfer point and to have a distance from one another of less than about 70 mm.
According to a feature of the instant invention, the fibrous web can include one of a paper, cardboard, and tissue web.
In accordance with another feature of the invention, the distance between the transfer elements at the transfer point may be greater than zero and smaller than about 35 mm.
Further, at least one of the transfer elements can include a roll, and the roll may include a dryer cylinder.
Still further, at least one of the transfer elements can include a belt, and the belt of the at least one transfer element may include a drying wire. Further, a suction element may be included, and the at least one transfer element can be the accepting transfer element, and the belt of the accepting transfer element may be arranged, at the transfer point, to be guided over the suction element. The suction element may include a suctioned roll.
Moreover, the transfer elements can include a roll and a belt, and the belt is arranged so as not to wrap around the roll at the transfer point.
According to a further feature of the present invention, the transfer elements may include belts arranged to travel together for a distance, as short as possible, at the transfer point.
In accordance with still another feature of the invention, a first dryer group may have only one or two dryer cylinders. Further, the first dryer group can include only one dryer cylinder.
The transfer point may be arranged between a first dryer group and a second dryer group, such that the speed difference between the transfer elements is arranged between the first dryer group and the second dryer group. The speed of the accepting transfer element at the transfer point may be maximally about 2.5% higher than the speed of the delivering transfer element. Further, the speed of the accepting transfer element at the transfer point can be between about 0.5 to 2.0% higher than the speed of the delivering transfer element.
The distance between the transfer elements may be greater than a thickness of the web and less than about 35 mm.
The present invention is directed to a process of producing and/or processing a fibrous web in an apparatus that includes a dryer section having a plurality of dryer groups and a transfer point, where each dryer group includes at least one heated dryer cylinder and at least one belt, where at least one dryer group further includes guide rolls, and where the transfer point is formed between transfer elements arranged a distance from each other of about 70 mm. The process includes guiding the fibrous web to a first dryer group, pressing, with the at least one belt, the fibrous web against jacket surfaces of the at least one heated dryer cylinders of each dryer group, and transferring, without pressing, the fibrous web between the transfer elements.
According to a feature of the present invention, the distance between the transfer elements at the transfer point may be greater than zero and smaller than about 35 mm.
Further, the distance between the transfer elements at the transfer point may be greater than a thickness of the fibrous web and smaller than about 35 mm.
In accordance with another feature of the invention, the transfer elements may include a roll and a belt, in which the belt is arranged so as not to wrap around the roll at the transfer point.
According to still another feature of the instant invention, the transfer point may be located between a first dryer group and a second dryer group.
Moreover, the transfer elements may include an accepting transfer element and a delivering transfer element, and the process can further include driving the accepting transfer element at the transfer point at a speed which is maximally about 2.5% higher than a speed of the delivering transfer element. The accepting transfer element at the transfer point can be driven at a speed between about 0.5 to 2.0% higher than the speed of the delivering transfer element.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.